A need exists to gather information regarding the functionality of printheads in terms of nozzle integrity, including the detection of non-functional or dead nozzles. Such information is critical during production phases for initial printhead calibration and also, more importantly, during key technology developmental stages and recalibration phases. In certain high-end commercial printers, it may also be desirable to provide information on dead nozzles during use without resorting to very high resolution scanning technology. Providing fast, robust, scalable yet affordable approaches to ascertain the aforementioned nozzle integrity information are essential to successful inkjet technological advancement.
Dead nozzles are typically detected by printing a specially designed pattern onto a sample of print media. The printed media is then digitized using an electronic imaging device, such as a charge-coupled device (CCD) line scanner, to form an image of the printed pattern. Finally the image of the pattern is analysed to extract the appropriate information. However, prior art methods are generally limited in terms of speed, cost, scalability and/or reliability.
FIG. 1 shows an image of an example pattern used for detecting dead nozzles. Arrow 100 indicates the direction of printing. The example pattern is formed by dividing the nozzles of the printhead in groups, and then controlling a single nozzle from each group to print a line segment having a predetermined length, such as line segment 101. After the single nozzle from each group has completed its line segment, a next neighbouring nozzle from each of the groups is controlled to each print another line segment, and so on, until all the nozzles of the printhead have printed a respective line segment. In the example pattern shown in FIG. 1 a space, such as space 102, is left between line segments printed by successive neighbouring nozzles to assist in discriminating between the line segments printed by respective nozzles. Furthermore, due to the fact that only one nozzle in each group prints at any one time, the line segments are separated in a direction transverse to the direction of movement, such as separation 103. The separation 103 is determined, to a large extent, by the resolving characteristics of the imaging device used to analyse the test pattern.
As is evident from the example pattern shown in FIG. 1, the pattern is spatially sparse and includes a large amount of blank space. Since the blank space contains no information, the example pattern, and other similar patterns, may be considered inefficient and require imaging of a large area of the page to gather the requisite dead nozzle information.
Perhaps a more significant deficiency of the example pattern shown in FIG. 1 is that the printhead is driven in an unconventional and unrealistic state; while a particular nozzle prints its line segment, none of its neighbouring nozzles are printing. Some print artifacts (e.g. those arising from poor nozzle chamber refill rates) are only apparent when groups of neighbouring nozzles are printing simultaneously. Hence, the example pattern shown in FIG. 1 may fail to detect some malfunctioning nozzles in a realistic printing scenario.
Still referring to FIG. 1, the existence of a dead nozzle is indicated by the absence of a line segment 101, such as in area 104. Current approaches share a similar methodology for establishing the presence of a line segment by quantifying the amount of deposited ink on the media at a sampled position within the pattern. However, those methods are vulnerable to interferences e.g. droplet misdirections or “keep-wet-spitting” 105 where nozzles are intermittently driven to eject ink and prevent nozzle dehydration (see, for example, U.S. Pat. No. 7,246,876, the contents of which are herein incorporated by reference).
A difficulty experienced after identifying an area 104 where the line segment is absent, is to determine which nozzle in the printhead is defective. To assist in identifying the defective nozzle, a number of registration marks/fiducials are printed alongside the pattern. FIG. 2 shows an example pattern 201 including registration marks/fiducials 202 and 203. Processing of the registration marks/fiducials 202 and 203, and using the registration marks/fiducials 202 and 203 to identify defective nozzles add significantly to the overall processing, and also further add to the inefficiencies already existing in the pattern.
It would be desirable to provide a method of identifying defective nozzles in a printhead, which is fast, reliable, and scalable to printheads having large numbers of nozzles, such as pagewidth printheads.
It would further be desirable to provide a method of identifying defective nozzles in a realistic printing state of the printhead, where neighbouring nozzles are fired simultaneously. In the present context, “fired simultaneously” is taken to mean “fired within one line-time”, one line-time being the time allocated to a row of nozzles to print one line of an image.